Personnel operating a wide variety of vehicles must be insulated from excessive heat and noise gen-erated by such vehicles. Delicate equipment also must be insulated. It is a regrettable fact that both man and machine may be exposed to nuclear, biological or chemical warfare. The severe heat and noise generated by combat vehicles and the conditions of such warfare combine to place rigorous demands on thermal/acoustical insulation. Although the insulating structures of the subject invention may be used in less demanding systems, they will be discussed in the context of those most extreme circumstances.
Fiberglass often is selected for insulation because of its low initial cost, availability in a wide range of thicknesses, and its good thermal and sound attenuation properties. Conventional fiberglass, however, has several undesirable properties which must be overcome.
Conventional fiberglass typically comprises a highly lofted layer of bonded fiberglass mounted on a paper backing. While air does not flow freely through the structure, some circulation does occur and, in fact, conventional fiberglass acts more in the nature of an air trap or circulation constraint. Thus, the structure of conventional fiberglass is best described as a circulating, continuous matrix of air in which glass fibers are dispersed.
Conventional fiberglass generally has high loft, i.e., low density, because for a given mass of fibers, as the loft is increased the relative proportion per volume of the better thermal conductor, glass, is reduced in favor of the poorer thermal conductor, air. At higher lofts, therefore, the overall thermal conductivity per unit volume will be decreased. Assuming no air circulation, the fiberglass also will have a higher insulating value.
Thus, relatively thick layers of conventional fiberglass are required for adequate thermal insulation. Added thickness can be a problem, however, in combat vehicles where space constraints are severe.
Reducing the loft does decrease air circulation, and thus, tends to increase the insulating value of the fiberglass. That increase, however, does not offset the corresponding decrease caused by increasing the relative proportion per volume of glass to air. Thus, its loft must be maintained within relatively narrow limits if the optimum insulating value of conventional fiberglass is to be realized.
Conventional fiberglass, however, compresses easily under load, and under vibrating conditions its fibers tend to loosen, break and settle. Vibrating conditions may create problems in attaching the fiberglass as well.
The threshold heat resistance of fiberglass is limited to that of its binder resin. That threshold generally is in the vicinity of 300 degrees Fahrenheit (.degree. F.). Turbine engines in tanks and similar vehicles, however, can create wall temperatures as high as 500.degree. F. or even higher. Under such conditions the conventional binder will flow or decompose and thereby compromise the structural integrity of the fiberglass.
Modern aircraft create an extremely noisy environment. Often the sound levels will be particularly high for certain frequencies. Because of its structure, conventional fiberglass cannot be varied to attenuate specific sound frequencies or a particular pattern of frequencies produced by different engine power levels or by different types of noise-generating equipment.
Fiberglass also can become saturated with oil fumes and thereby create potential fire hazards. Such hazards are particularly acute under extreme thermal conditions, especially where the possibility of flame or sparks exists.
During nuclear, biological or chemical warfare, the insulation may be exposed to a variety of chemicals. Fiberglass absorbs many chemicals that cannot be washed out, nor can it be decontaminated by steam. Water or steam pressure breaks up its fiber structure and degrades its insulating properties.
The subject invention, therefore, is directed to insulating structures which effectively attenuate heat and sound at reduced thicknesses. It also is directed to thermal/acoustical insulating structures having increased resistance to compression, flexibility for comforming it to irregular surfaces and/or structural integrity under vibrating conditions. It is directed as well to thermal/acoustical insulating structures which can be mounted on surfaces as hot as 500.degree. F. It is directed further to thermal/acoustical insulating structures which selectively attenuate particular sound frequencies or distributions of sound frequencies. Finally, it is directed to non-absorbing thermal/acoustical insulating structures, the surfaces of which can be easily washed and decontaminated with no impairment of insulation properties.